Tungsten seal coat



TUNGSTEN SEAL COAT James E. Webb, Administrator of the National Aeronautics and Space Administration, with respect to an invention of James R. Lindgren, San Diego, Alfred J. Weinberg, El Cajon, and Robert A. Holzl, La 'Canada, Calif.

N Drawing. Filed Dec. 27, 1966, Ser. No. 605,088

Int. Cl. C23c 13/02 US. Cl. 117107.2 1 Claim ABSTRACT OF THE DISCLOSURE A two-step process for cladding nuclear fuels with tungsten, comprising first vapor depositing a thin seal coat of tungsten onto the fuel element in an evacuated chamber under reaction conditions suitable for the application of such a thin coat. This is followed by the vapor deposition of a final heavier main outer coating of tungsten over the seal coating under conditions more suitable for such cladding.

The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 USC 2457).

This invention relates to a process for depositing tungsten onto chemically reactive materials, particularly nuclear fuels.

Various techniques have been tried, and several processes have been investigated for coating nuclear fuels to produce a barrier against fission products and provide a nonreactive substrate. Compatibility studies have established pure tungsten is the most suitable material for use as a direct cladding for nuclear fuels such as uranium carbide (UC), uranium carbide-zirconium carbide (UC-ZrC) tungsten-uranium carbide (W-UC) cermet, and tungstenuranium oxide (W-UO cermet compositions, the fuels being considered as candidates for thermionic generators to be operated at temperatures up to 1800 C. Tungsten is a relatively difficult material to fabricate using conven tional techniques and procedures. One of the conventional techniques explored for cladding nuclear fuels involved machining materials to provide a sleeve or cut-type container. This technique was not particularly satisfactory since a space between the sleeve and fuel existed which allowed for leakage of the fission products. The brittle physical properties of tungsten also contributed to the failure of this technique.

Various conventional chemical deposition processes were investigated, as aforementioned, but because of vigorous chemical reaction, the fuels were largely consumed before a suitable barrier deposition was achieved. For instance, attempting to deposit tungsten in a conventional manner will result in the fuel reacting vigorously with the deposition components reducing the fuel to an unacceptable powder.

However, the foregoing problems are, in general, solved with the present thermochemical techniques which may be considered a two-step or two-phase process, the initial operation being to locate the nuclear fuel in an evacuated chamber. Thereafter, phase one concerns depositing a relatively thin seal coat of tungsten over the entire fuel specimen to thereby reduce, or prevent, the vigorous chemical reaction which would otherwise occur between the fuel and a component or a by-product of the deposition process. To achieve deposition of a satisfactory seal coat, the speed of chemical reaction is controlled by limiting or controlling the deposition temperature and the initial quantities of used chemical components.

3,501,337 Patented Mar. 17, 1970 The second and final step, or phase, of the process is carried out under more ideal conditions. The vigorous chemical reaction between the fuels and chemical byproduct is controlled to such an extent by the seal coat, a relatively heavy deposition of tungsten may be applied to the fuels at temperatures in excess of those employed in phase one, and the use of chemical components may be more liberally used without the occurrence of adverse results.

Depositing one metal onto another is quite common and well known, such as depositing copper onto aluminum, nickel or chromium plating of another material. However, these processes do not necessarily involve a vigorous chemical reaction occurring between the material plated and a component or by-products of the process. As a result, it is unnecessary to provide a seal coat to control the reaction, and thereafter apply a relatively heavy deposit of material. The prior art processes are acceptable when vigorous chemical reactions can be ignored, but they are not acceptable when chemical reaction is significantly a part of the technique.

A knowledge of the aforementioned nuclear fuels indicates they are extremely reactive in moist atmospheres, and in the presence of contaminants such as oxygen and reaction is accelerated by increasing or rising temperatures. At even moderate temperatures oxygen vigorously attacks the fuels. Thus, it may be seen, substantially all sources of contamination are required to be removed from the plating atmosphere by suitable purging with a substantially inert material and the temperature is to be rigorously controlled in order to deposit tungsten onto nuclear fuels. Once the source of contamination are removed and the temperature suitably controlled, a seal coat of tungsten is deposited onto the fuel specimen under conditions favorable for minimum reactions to occur between fuel and plating atmosphere. After deposition of the seal coat, the specimen may be treated as an ordinary piece of tungsten which is not deleteriously affected by the corrosive atmosphere, even at much higher temperatures. The seal coat, therefore, establishes a condition whereby a heavier coat of tungsten may be deposited under more favorable conditions.

In order to provide a contamination-free atmosphere for depositing a seal coat of tungsten onto a fuel specimen, the unit is placed in a plating chamber which is evacuated by a mechanical vacuum pump. During the actual deposition phase, which will hereinafter be described in more specific detail, the chamber is purged, and a water aspirator is employed to exhaust reactants. During this operation, a vac uum of between about 5-380 mm. of mercury (Hg) absolute produces satisfactory and acceptable deposits of a tungsten seal coat.

Applicable tungsten compounds for achieving a deposition thereof, in both phases one and two of the process, are tungsten halides and tungsten carbonyls.

The procedure for depositing a seal coat using tungsten hexachloride (WCI will be initially considered, and as prevoiusly noted, the plating chamber is to be evacuated with a mechanical vacuum pump, the pressure within the chamber being about 12-20 mm. Hg absolute.

Thereafter, a greater than stoichiometric or rich amount of hydrogen (H is admitted into the chamber, the volume found to be most acceptable being about 3.4 liters per minute.

Following admission of H into the chamber, less than a stoichiometric quantity of WC1 is admitted, the volume being about 0.3 liter per minute, and the ratio of H to WCl ranging between about 6:1 to 10:1, the optimum ratio being about 7.5: 1.

With H and WCl admitted to the plating chamber, the temperature therein is elevated to between about 8001100 C., with the optimum deposition temperature being about 9001100 C. Carefully maintaining the temperature for about forty-five minutes, which will provide an acceptable tungsten seal coat of about 0.003 to 0.004 mil thick. The actual plating operation produces HCl which is reactive with nuclear fuels, but this compound is effectively removed by the water aspirator apparatus.

After deposition of the seal coat is achieved, the temperature is lowered, admission of V/Cl into the chamber is stopped, the sealed specimen cools in the H stream, and when cooled, admission of H is stopped. During the cooling, the pressure of the chamber can be atmospheric in order to increase the cooling rate. Thereafter, the chamber is again evacuated and the water aspirator apparatus is stopped. The specimen of nuclear fuel is now in condition to be encapsulated in a heavier coat of tungsten.

Encapsulation, phase two of the sealed specimen, is carried out using tungsten hexafluoride, WF only using a carefully controlled lower temperature of about 700 C. The primary reason for carrying out the encapsulation phase using WF rather than WCl is that deposition rate is higher for WF If WCl were to be used for phase two, the conditions would be the same as those used during the application of the seal coat.

Regardless of the initial tungsten compound employed for obtaining deposition of tungsten onto the specimen, phase two of the process is substantially the same.

The procedure for depositing a seal coat of tungsten onto the nuclear fuels from tungsten hexafiuoride, WF is basically the same as that when WC is employed. However, because of the activity of formed hydrogen fluoride, some variations are applicable: helium purge is required to remove substantially all contaminants, sealing temperatures are lower to control reaction, and ratios of H and tungsten halide is altered to achieve an effective seal coat, all of which will hereinafter be considered in more specific detail.

After the plating chamber is evacuated initially, and prior to actually depositing the seal coat layer of tungsten from WF onto the specimen, the plating chamber is back-filled or purged about six or more times with inert helium (He) to dilute any residual contaminants such as oxygen. However, to avoid introducing contaminants into the plating atmosphere through the helium, the purging gas is purified by passing it through an activated charcoal filter and liquid nitrogen trap which effectively removes oxygen and water. Thereafter the chamber is again evacuated to remove the helium. Then about 380 mm. Hg absolute of purging helium is again admitted into the plating chamber, and at the same time activation of the water aspirator vacuum apparatus is initiated to achieve exhausting of the HF. To assure reliable purging of the chamber, a flow of between about 4-5 liters per minute of He is provided. If the plating chamber is not adequately purged, undesirable reactions will occur which will adversely effect the final product.

Following the admission of hydrogen into the plating chamber, WF is admitted, the volume of H being about 2.4 liters per minute, and the ratio of H to WF should range between about 6:1 to 10:1, the optimum ratio being about 8: 1. This relatively high ratio of H to WF is necessary in order to achieve an adequate seal coat. Failure to employ the high ratio will materially reduce to possibilities of achieving that which is desired. Even a minor reduction in the ratio will increase the difficulties of acquiring a seal coat.

With H and WF in the plating chamber, the temperature therein is elevated to between about 200-400 C.,

with the optimum deposition temperature being about 350400 C. Attention is again directed to the relatively low temperatures employed which must be rigorously maintained in order to obtain optimum results.

The removal of formed hydrogen fluoride, which vigorously reacts with nuclear fuels, is effectively achieved by the water aspirator apparatus and the flow-by of the He, the inert gas, in effect, blowing the HF away from the specimen. As before mentioned, if the plating is not adequately purged, it may be seen HF would be free to attack the specimen.

Phase two, the encapsulation operation, is the same as that previously described.

As aforementioned, tungsten pentabromide (WBr is also applicable for applying a seal coat to nuclear fuel specimens, and the procedure is much the same as that which was used for WCI except the seal coat temperature ranges between about 7501000 C.

Again the general procedures which were applicable when WF was used is also applicable when tungsten carbonyl W(CO) is employed, but again with some exceptions. The seal coat deposition temperature should be about 350 C. and the flow rate of W(CO) should be about 0.05 liter per minute. Helium purge is also used, but the fiow rate is about 0.1 liter per minute, and the plating chamber pressure ranges between about 5-10 mm. of mercury absolute. One other difference exists between the method employed when W(CO) is used and when WF is used, and that is no hydrogen is admitted into the plating chamber.

What is claimed is:

-1. A process for depositing tungsten onto nuclear fuels which comprises:

(a) locating said nuclear fuels in a chamber subsequently evacuated to about one-half atmosphere;

(b) introducing into said chamber to thereby obtain a seal coat of tungsten onto said nuclear fuels:

(1) a quantity of tungsten hexachloride;

(2) a greater than stoichiometric quantity of hydrogen to reduce the tungsten hexachloride, the ratio of hydrogen to tungsten hexachloride ranging between 6:1-10:1;

(c) heating said chamber to between 800 to 1100 C. to deposit a coat of 0.003 to 0.004 mil thickness of tungsten onto said nuclear fuels;

(d) letting the sealed material cool;

(c) then introducing into said chamber:

(1) tungsten hexafiuoride, and

(2) an amount of hydrogen sufiicient to reduce the tungsten hexafiuoride to produce hydrogen fluoride to thereby deposit a thicker coat of tungsten onto said nuclear fuels, and

(f) heating said chamber to about 700 C. to effect the deposition of said thicker coat.

OTHER REFERENCES Powell et al.: Vapor Plating, 1955, pages 55 to 58 relied upon.

ANDREW G. GOLIAN, Primary Examiner US. Cl. X.R. 17682 

